1. Field of the Invention
The present invention relates to an image forming apparatus for forming a color image on a recording material, and more particularly, it relates to a recording apparatus as an image output device for a computer and a copying apparatus for a color original.
2. Related Background Art
In image forming apparatuses, particularly, in copying machines and printers, which have been widely used in offices, it has been requested that the apparatus be made compact, be operated at a higher speed and obtain a high quality image.
Among image forming methods in the image forming apparatuses, there have been proposed an electrophotographic method and an ink jet method. In the electrophotographic method, for example, a toner image is formed on a photosensitive drum by a charge means, an exposure means and a developing means which are disposed around the photosensitive drum, and, after the toner image is transferred onto a recording medium (referred to as "recording sheet" hereinafter) by a transfer means, the transferred toner image is fixed to the recording sheet by a fixing means. This method can form a high quality image at a high speed. However, although mono-color image forming apparatuses utilizing such an electrophotographic method can be made compact (for example, desk-top type), color image forming apparatuses utilizing such an electrophotographic method causes a problem that they are made bulky and expensive.
On the other hand, in the ink jet method, an ink image is directly formed on a recording sheet by discharging ink droplets from a recording head. Although this method can be made compact, an image forming speed and image quality are worsened in comparison with the electrophotographic method in both mono-color image forming apparatuses and color image forming apparatuses. In particular, regarding character or letter images, when the character image is superimposed on the color image, there arises a problem that the character image is spread on the color image.
In order to solve the above problems, there have been proposed image forming apparatuses having both an image forming means utilizing an electrophotographic method and an image forming means utilizing an ink jet method, as disclosed in the Japanese Patent Application Laid-open Nos. 4-294379, 5-6127 and 5-134824. In such image forming apparatuses, the image forming methods are switched appropriately so that mono-color images mainly including character images which are frequently required in offices are formed by the electrophotographic method and color images which are not frequently required in offices are formed by the ink jet method. By switching the image forming methods in this way, the optimum image forming apparatus can be used in each office. That is to say, in the image forming apparatus including both of the above-mentioned methods, the installation space in the office can be reduced, the mono-color image can be obtained with high quality at a high speed and the color image can be obtained, if necessary.
However, in the above-mentioned conventional image forming apparatuses, since a dimension of the recording sheet is changed after the image formation is effected by the preceding image forming method, in the succeeding image forming method, even when registrating means such as regist rollers and guides are used, the mechanical position of the recording sheet will be deviated by a certain amount. As a result, when the image formation is effected by the succeeding image forming method, there arises deviation between the image formed by the preceding image forming method and the image formed by the succeeding image forming method, thereby deteriorating image quality. Particularly, when the preceding image forming method is the electrophotographic method, since the transferred toner image is permanently fixed to the recording sheet by heat and pressure, the recording sheet is contracted or shortened. Accordingly, the toner image fixed to the recording sheet is also contracted or shortened. As a result, the image formed by the succeeding image forming method is deviated from the fixed toner image, thereby worsening the image quality.